FOUS30 KWBC 260802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Tue Aug 26 2025

Day 1
Valid 12Z Tue Aug 26 2025 - 12Z Wed Aug 27 2025

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN ARIZONA, THE NORTHERN GREAT BASIN, THE SIERRA NEVADA, THE
CENTRAL ROCKIES AND THE SOUTHERN PLAINS...

...The West...

The sprawling upper ridge over the Southwest continues to funnel
rich monsoonal moisture northward through the Intermountain West. 
ECMWF guidance shows PWATs in excess of 1" advancing as far north 
as the Blue Mountains and Columbia River Valley of WA/OR. Some 
PWATs are also expected to top 1.25" in portions of the Great Basin
and Snake River Valley. Helping to add some upper-level support 
aloft will be the presence of a meandering 700mb low positioned 
over the Pacific Northwest and a healthy diffluent pattern at 250mb
thanks to a longwave trough situated along the West Coast. With 
such high moisture content, not much in the way of surface based 
heating will be needed to spark widespread thunderstorms Tuesday 
afternoon and through Tuesday evening. Combined with MUCAPE between
500-1,000 J/kg, thunderstorms will be capable of generating up to 
1.5"/hr rainfall rates, which will pose a flash flood threat in 
areas of complex terrain, burn scars, and nearby creeks and 
streams. The Slight Risk over the northern Great Basin and 
portions of the Snake River Valley remains in place. The Slight
Risk area resides within the strongest signal from the ECMWF-EFI
with 0.8-0.9 values, which give support to highly unusual QPF
totals and potential impacts for this time of year. A Slight Risk
was also introduced over the central Sierra Nevada given the
lingering threat for flash flooding over the rugged terrain that is
growing increasingly more saturated and sensitive.

...Southern Arizona...

Fresh off a busy day of thunderstorms on Monday, there is the
possibility that the atmosphere is overworked to where converage of
thunderstorms is not as expansive as Monday. However, southeast AZ
was not as active on Monday and should tap into modest instability
(500-1,000 J/kg of MUCAPE) and moisture aloft (PWATs up to 1.5" in
some cases). This continues to support rainfall rates up to 2"/hr 
in the more intense storms. While storms will remain more pulse- 
like in nature, these potent storms still pose a flash flood 
threat, especially in dry washes and in spots that drain poorly.

...Central & Southern Rockies...

Guidance across the board shows a higher concentration of
atmospheric moisture over the region on Tuesday compared to Monday,
and that is notable considering PWATs were already above the 97.5
climatological percentile on Monday. PWATs in the tallest peaks
will generally hover near 1" at their highest values, but along and
just east of the Front Range, PWATs are likely to range between
1.3-1.5". These values are surpassing the 99th climatological
percentile while also having 250-500 J/kg of MLCAPE present. With
soils steadily growing more and more saturated due to recent heavy
rainfall and the favorable atmospheric parameters in place, more 
instances of flash flooding are expected with complex terrain and 
burn scars more at-risk.

...Southern Plains...

A nearby frontal boundary will be responsible for triggering
thunderstorms this morning in portions of central OK. Following a
brief lull in the action this afternoon, a rejuvenated LLJ will
cause low-lvel WAA and additional storms to fire along the warm
front over the OK/TX Panhandles. PWATs are expected to top 1.8"
(also above the 90th climatological percentile) throughout the day
and into Tuesday night, while some meager MUCAPE values topping out
around 500 J/kg are expected. The 00Z HREF guidance shows low-
chance probabilities (20-30%) for rainfall >2" across central and
southern OK, but soils have been gradually moistening following
recent days of heavy rainfall in the area. Given these factors, the
inherited Slight Risk remains in place with instances of flash
flooding most likely to occur in poor drainage areas and locations
with more sensitive soils.

...Western Florida Peninsula...

A similar setup to Monday is expected to produce thunderstorms
capable of producing torrential downpours over the heavily
urbanized western Florida Peninsula today. An approaching cold
front will act to trigger thunderstorms in an atmospheric
environment that contains up to 2.25" PWATs and MLCAPE around 
1,000 J/kg. Surface-500mb mean winds remain fairly uniform out of 
the WSW, so there is the potential for training thunderstorms 
around the Tampa-St. Pete metro area. The 00Z HREF shows moderate-
to-high chances (50-70%) for 24-hour rainfall totals >3" and low 
chances (10-30%) for 24-hour rainfall totals >5". The inherited 
Marginal Risk remains on track with the more urbanized areas 
containing the greatest concentration of hydrophobic surfaces most 
at-risk for flash flooding. 

Mullinax
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